1. Field of the Invention
This invention relates to a signal processing method for processing video signals.
2. Description of the Related Art
The video cameras which are now rapidly becoming popular as a result of the recent achievement of reduction in size and increase in recording density are taken up by way of example in the following description of the present specification.
Analog-to-digital (hereinafter referred to as A/D) and digital-to-analog (hereinafter referred to as D/A) converters which are capable of operating at a high speed have recently become practicable for video signal processing within a video camera as a result of the advancement of semiconductor technology. It has been, therefore, proposed to change the video signal processing action of the video camera from the conventional analog signal processing to digital signal processing.
The video camera which is thus arranged to carry out digital signal processing is described below with reference to FIG. 1, which shows it in a block diagram:
In shooting and recording an image, an image is formed through a lens which is not shown. The image is photo-electrically converted by a CCD 51 into a pickup image signal. This signal is sent via a correlated double sampling circuit (hereinafter referred to as CDS) 52 and an automatic gain control circuit (hereinafter referred to as AGC) 53 to an A/D converter 54 to be converted from its analog state into a digital signal.
The digital signal is sent to one-horizontal period delay circuits (hereinafter referred to as 1HDL circuits) 55 and 56 to be delayed by them as much as one horizontal period, respectively. The delayed signals thus obtained come to a camera luminance (Y) processing circuit (hereinafter referred to as camera Y-processing circuit) 57 and a camera chrominance (C) processing circuit (hereinafter referred to as camera C-processing circuit) 58. The camera Y-processing circuit 57 forms a luminance signal from the digital-converted pickup image signal. The luminance signal is supplied to an adder 59 to have a synchronizing (hereinafter referred to as sync) signal added thereto. With the sync signal added, the luminance signal is sent to a recording/reproduction Y-processing circuit 68. The recording/reproduction Y-processing circuit 68 then performs various processes, such as an emphasizing process, etc., that are necessary for magnetic recording. The output of the circuit 68 comes to a D/A converter 69 to be converted into an analog signal. The analog signal is supplied to a frequency modulator 70 to be frequency-modulated.
Meanwhile, the camera C-processing circuit 58 forms two color-difference signals, i.e., R-Y and B-Y signals, from the digital-converted pickup image signal. The two color-difference signals are modulated into a chrominance signal. The chrominance signal is sent to a recording/reproduction C-processing circuit 71. The recording/reproduction C-processing circuit 71 then down-converts the frequency of the chrominance (C) signal, i.e., converts it into a low frequency band. The output of the C-processing circuit 71 is converted into an analog signal by a D/A converter 72.
The Y signal which has been frequency-modulated and the C signal which has been low-frequency-converted (or down-converted) are added together by an adder 73 into a composite video signal. The video signal is recorded on a video tape 86 by a recording/reproducing head 76 through a recording amplifier 74 and a change-over switch 75.
In a case where an external input image is to be recorded and the image is inputted in a state of being divided into a luminance signal and a chrominance signal, the video camera performs recording actions as follows:
The luminance (Y) signal which is included in the input from an external apparatus (not shown) is converted into a digital signal by an A/D converter 60. The digital signal is sent via a change-over switch 66 to the recording/reproduction Y-processing circuit 68. The signal processing action of the circuit 68 and the signal processing actions to be performed thereafter are the same as the signal processing actions to be performed in recording the pickup image signal as described above.
The chrominance (C) signal which is included also in the input signal from the external apparatus is converted into a digital signal by an A/D converter 62. The digital signal is sent via a change-over switch 67 to the recording/reproduction C-processing circuit 71. The signal processing actions to be performed by the circuit 71 and by other circuit elements after the circuit 71 are the same as the signal processing actions to be performed in recording the pickup image signal as described above.
Next, in recording an external input image which comes in a state of a composite video signal, the video camera performs recording actions as described below:
The input composite video signal from the external apparatus which is not shown is converted into a digital signal by an A/D converter 61. The digital signal is sent to a Y/C separation circuit which is composed of a 1HDL circuit 63, an adder 64 and a subtracter 65. The Y/C separation circuit separates the digital signal into a luminance signal and a chrominance signal. The luminance signal is sent via the change-over switch 66 to the recording/reproduction Y-processing circuit 68. Meanwhile, the chrominance signal is sent via the change-over switch 67 to the recording/reproduction C-processing circuit 71. The processes to be performed by the Y-processing and C-processing circuits 68 and 71 and by other circuit elements after the circuits 68 and 71 are the same as the processes to be performed in recording the pickup image signal.
The processes to be performed by the video camera for reproduction are as described below:
A signal recorded on the video tape 86 is read out by the recording/reproducing head 76. The output of the recording/reproducing head 76 comes via the change-over switch 75 to a head amplifier 77 to be amplified up to a given level. The reproduced signal thus amplified comes to a high-pass filter (hereinafter referred to as HPF) 78 and a band-pass filter (hereinafter referred to as BPF) 80. A frequency-modulated luminance signal is separated by the HPF 78. The luminance signal is demodulated by a demodulator 70 and converted into a digital signal by an A/D converter 79. The digital signal is sent to the recording/reproduction Y-processing circuit 68 to be subjected to processes which are performed in a manner reverse to the processes performed in recording.
A down-converted chrominance signal is separated by the BPF 80. The chrominance signal is sent to the recording/reproduction C-processing circuit 71 to be subjected to processes which are performed in a manner reverse to the processes performed in recording.
The luminance and chrominance signals which are thus converted into their original states are converted into analog signals respectively by D/A converters 82 and 83 in the state of being separated from each other. These analog signals are outputted either as they are from Y and C output terminals of a part LINE OUT or combined together by an adder 84 to be outputted as a composite video signal from a composite output terminal of the part LINE OUT.
Further, the luminance signal which is converted into an analog signal by the D/A converter 82 is outputted also to an electronic viewfinder (hereinafter referred to as EVF) 85, so that it can be used for monitoring.
With respect to the video signal processing, various processing circuits are necessary for a high picture quality and a high resolution in addition to the circuits for fundamental signal processing. For example, although the down-converted chrominance signal does not interfere directly with the frequency-modulated luminance signal, the carrier wave "fo" of the luminance signal and the frequency "fL" of the down-converted chrominance signal bring forth a signal component of a frequency "fo-2fL", which overlaps the luminance signal to lower the picture quality at the time of magnetic reproduction. Therefore, a filter is added to attenuate the frequency component "fo-2fL" (hereinafter referred to as the suppression of a Y-2C component) for improvement in the picture quality of the reproduced signal. Further, a contour emphasis circuit for emphasizing the contour of the reproduced luminance signal and a time delay circuit for correcting a phase difference between the reproduced luminance signal and the reproduced chrominance signal are arranged after the D/A converter 82 for attaining a high picture quality.
However, any arrangement to digitally carry out the above-stated signal processing operation of the video camera necessitates the provision of an A/D converter in the signal input part and a D/A converter on the side of signal output. Therefore, although the details of the signal processing operation are equivalent to those of the analog signal processing operation, the number of necessary component parts is increased by as much as the parts added for digital processing. Further, in the digital processing circuit arrangement, each of image pickup, recording and reproducing systems are also arranged to use circuits which are arranged discretely from those of other systems. In addition to this, the provision of the circuits for a high picture quality causes a further increase in scale of the circuit arrangement, which is disadvantageous in respect to a reduction in cost and size.